Those are lice eggs. Couple of things to do right away. First, don't panic--it is totally curable, just a bit of a pain. Diatomaceous earth is a wonderful preventative, but really not strong enough once there is an infestation. Sevin has been banned for poultry use, although many use it. I recommend using Bonide Eight Insect Control and Garden Dust. It is still an insecticide, so precautions need to be taken. Wearing gloves and sticking a bandana over your mouth/nose is not a bad idea. You can get bottles with a spout, which makes dusting a bit easier. Lay the hen on her side/back to get her calm. You can even have someone else cover her head lightly with a towel to calm her. Dust throughly, making sure to get the dust under the plummage. Do near the vent, but NOT in the vent. I also (carefully) cut the egg infested feathers out and dispose of them far away. You'll also need to clean out the whole coop. A layer of Dry Stall (it contains diatomaceous earth and controls wetness/odor) on the bottom is a good idea. Also spraying or painting mineral oil over all surfaces smothers any live louse in the coop. You need to reapply the Bonide a week later. I've heard of it taking longer, but I find one or two treatments is all that it takes. Most important the first day is to treat the birds. If you've had it, save cleaning the coop till tomorrow, since the girls will be covered with the Garden Dust. This too shall pass!